Various schemes have been developed and used for fluid level detection. Non-optical type sensors such as traditional wired systems have been used widely but they are relatively expensive, usually difficult to assemble, and have several other disadvantages. For example, signals passing through wires often cause cross talking with signal transmission in adjacent wires. Signals passing through wires also cause electromagnetic interference in adjacent wires unless some type of shielding is used. These interferences cause signal distortion.
Optical sensors offer a number of advantages over non-optical sensors. For example, optical sensors do not require electrical contact with the fluid to be monitored. Thus, optical sensors can be fabricated using relatively simple designs. In addition, they are easier to fabricate because they do not have moving parts or components. Also, there exists a variety of materials that may be used for signal transmission that are nonreactive or resistant to the fluid the volume or level of which is to be measured.
On the other hand, while optical fibers are sometimes used in place of wires or incorporated in a molded structure, the use of optical fibers increases the cost of the electronic component system. In addition, integration of optical and electrical components is usually not suited for high volume manufacturing because of difficulties in assembly.
The measurement of the fluid level using optical sensors is typically based on the difference of the indices of refraction of two media that are in contact with each other, e.g., an optical fiber and a fluid in which the optical fiber is immersed, or an optical fiber and the air that surrounds the optical fiber.
The measured intensity of the reflected beam that eventually reaches the sensor depends on the fluid level because a fluid has a different index of refraction from a medium such as air. Because the amount of light that leaks at the interface of two media depends on their relative indices of refraction, different fluid levels give rise to correspondingly different amounts of light that leaks at the interface because, for example, higher fluid levels means a greater portion of a medium such as an optical fiber is surrounded by the fluid rather than air. Thus, compared to a lower fluid level, a higher fluid level would result in a different amount of reflected energy that a sensor detects.